The present invention relates to a circuit arrangement for the correction of a sensor output quantity. According to the invention, sensitivity changes of the sensor which effect the measurement result, are compensated, at least approximately, by electric circuit elements.
Electro-chemical measuring sensors, e.g. alcohol measuring cells, in which output quantities are produced on the basis of substance conversions, have in many cases, the disadvantage that the sensitivity of the sensor diminishes as a function of the concentration of the medium to be measured and its contact time. The resulting reduction in sensitivity may be caused either by an irreversible process, e.g. surface changes of the sensor, or it may be regarded, depending on the type of sensor, as a reversible fatigue process where the sensor regains its original sensitivity after some time outside the influence of the medium to be measured. In cases of electrolytic conversions which diminish the detection sensitivity of the sensor, a regeneration of the original sensor properties and hence of the original sensitivity can possibly be achieved bv pole reversal.
Sensitivity changes in sensors which depend on the concentration or contact time of the medium to be measured, produce, depending on the prior history of the sensor, more or less faulty test results which must be corrected by taking into consideration the known sensitivity changes.
A circuit arrangement for the correction of sensor output quantity has been proposed in German patent application No. P 29 29 387. In this application a device is disclosed for picking up the partial pressure of gases with electrode sensors using memory data of the activity ratio between initial activity of the freshly calibrated work electrode and the activity existing at the time of measurement. This activity ratio is observed at certain intervals of time, e.g. daily. The measurement quantity is modified multiplicatively, whereby a corresponding correction of the sensitivity changes becomes possible. The measuring method described appears relatively expensive because at fixed intervals program-controlled measuring sequences must be carried out for the determination of the activity ratio.